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styrene production
Housseinou Ba
To cite this version:
Housseinou Ba. Manufacturing of metal-free carbon-based catalysts for styrene production. Other. Université de Strasbourg, 2015. English. �NNT : 2015STRAF026�. �tel-01276351�
ÉCOLE DOCTORALE DES SCIENCES CHIMIQUES
ICPEES, UMR 7515 CNRS
UCCS, UMR 8181 CNRS
THÈSE
présentée par
Housseinou BA
soutenue le 24 Juillet 2015pour obtenir le grade de
Docteur de l’
Université de Strasbourg
Discipline/ Spécialité: Chimie /Chimie Matériaux
Développement des catalyseurs sans métaux à
base de carbone pour la production de styrène
Manufacturing of metal-free carbon-based catalysts for styrene
production
Membres du jury
Directeur de thèse : Dr. Cuong PHAM-HUU Directeur de recherche, UDS, Strasbourg
Co-Directeur de thèse :
Prof. Pascal GRANGER Professeur, UDL1, Lille
Rapporteur externe : Prof. De CHEN Professeur, NTNU, Trondheim (NO)
Rapporteur externe Prof. Alexei LAPKIN Professeur, UC, Cambridge (UK)
To my parents
To my wife
I would never have made this exciting experience without the help of a lot of people around me. I would like to give many thanks to all of them.
The researchs which are the subject of this thesis was performed at the Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES UMR 7515 CNRS and the University of Strasbourg) in Strasbourg.
First of all, I would like to give my sincere thanks to my supervisor, Dr. Cuong PHAM-HUU for accepting me in his laboratory, for his guidance, patience, and encouragement during my research. He is always ready to help me and guide me in the right scientific direction. I extend to him my warm and heartfelt thanks for the quality of his leadership and support. He is the funniest advisor and one of the smartest people I know. I am deeply grateful to him for making me benefit from his experience and expertise throughout the work. I have learned a lot from him, and without his help I could not have finished this dissertation successfully.
I would like to thank also my co-supervisor Prof. Pascal GRANGER, who accepted me as his Ph.D. student without any hesitation when Dr. Cuong PHAM-HUU presented him my research proposal. It was only due to his valuable guidance, cheerful enthusiasm and ever-friendly nature that I was able to complete my research work in a respectable manner.
I also want to thank Prof. Alexei LAPKIN from the Department of Chemical Engineering and Biotechnology at the University of Cambridge (UC) and Prof. De CHEN from the Chemical Engineering Department of the Norwegian University of Science and Technology (NTNU) for coming from a long way for serving as the advisory committee members. I would like also to express my thanks to Prof. Ovidiu ERSEN from the Institut de Physique et Chimie des Matériaux de Strasbourg (UMR CNRS 7504) at the University of Strasbourg who helped me for the TEM analysis and for accepting to be a member of the jury.
Special thanks are also given to Dr. Jean-Mario NHUT, Dr. Yuefeng LIU and Dr. Lai TRUONG-PHUOC, who also actively monitored the progress of my research. I would like to thank for their help, advice availability, and encouragement, which made me feel confident to fulfill my research and overcome every difficulty I encountered. At the last stage of my dissertation, they helped me to correct grammar mistakes in the dissertation text, and suggested possible improvements. It is not sufficient to express my gratitude with only a few words. I am also indebted to Drs. Dominique BEGIN, and Izabela JANOWSKA and Ioana FECHETTE for their valuable guidance and encouragement.
I warmly thank Mr. Cuong DUONG-VIET for his valuable support, availability, human values and attention he has shown to me throughout this work.
I would like to thank my friends in the ICPEES for their friendship, help and discussions: Fabrice VIGNERON, Drs. Tri TRUONG-HUU, Walid BAAZIZ, Seetharamoulou PODILA, and Xiaojie LIU.
I extend great thanks to all the administrative team (Véronique VERKRUYSSE, Agnès ORB, Nathalie WEBER and Francine JACKY) and the technical staff (Pierre BERNHARDT, Won-Hui DOH, Spyridon ZAFEIRATOS, Thierry ROMERO, Sécou SALL, Alain RACH, Thierry DINTZER, Christophe MELART, Christophe SUTTER…) of the ICPEES and the Faculty of Chemistry of Strasbourg for the help they have often brought me in my various approaches.
My sympathy also goes to trainees, Yann UBERSHLACH, Mathilde ILTIS, Camille HELLEU and Duo ZHANG, for their sympathy and friendship, encouraged me and helped me to carry out this work in excellent conditions.
I want to express my deep gratitude and thanks to my friends, who have shown me unwavering support in accomplishing this work. Their assistance is greatly acknowledged.
I have a special thought for my family of the sacrifices that they made on my behalf. Their prayers and support throughout the years for me was what sustained me thus far.
At the end I would like to express appreciation to my beloved wife Fatou TOURE who spent sleepless nights with me and was always my support in the moments when there was no one to answer my queries.
Acknowledgement……….………...i
Table Contents……….…..iii
Résumé ... 7
Références bibliographiques ... 19
CHAPTER 1 ... 21
1. Generalities about styrene ... 23
1.1. Physical properties of ethylbenzene ... 27
1.2. Physical properties of styrene ... 27
2. Industrial process for styrene production ... 28
2.1. The limitations of current industrial processes ... 33
3. Carbon-based composites as metal-free catalysts ... 34
3.1. Oxidative Dehydrogenation (ODH) process ... 37
3.2. Direct dehydrogenation (DH) process ... 42
4. Objectives of the work ... 49
Reference ... 51 CHAPTER 2 ... 57 ... 57 1. Materials synthesis ... 59 1.1. Nanodiamonds ... 59 1.2. Graphene based-materials ... 61
1.3. Carbone nanotubes / nanofibers ... 63
1.4. Silicon Carbide ... 66
2. Catalytic Reactions ... 69
2.1. Selective dehydrogenation of ethylbenzene to styrene ... 69
2.2. Oxygen Reduction Reaction (ORR) ... 70
3. Characterization techniques ... 71
References ... 74
CHAPTER 3 ... 77
Nanodiamond decorated few-layer graphene composite as an efficient metal-free dehydrogenation catalyst for styrene production ... 79
Graphical abstract ... 80
Abstract ... 80
1. Introduction ... 81
2. Experimental ... 82
2.1 Nanodiamonds and few-layer graphene characteristics... 82
2.2 Synthesis of the ND/FLG catalyst ... 83
3. Results and discussion ... 83
3.1 ND/FLG Characteristics ... 83
3.2 Steam-free selective dehydrogenation of ethylbenzene to styrene ... 87
3.3. The surface and structure analysis after dehydrogenation reaction ... 91
4. Conclusion ... 97
Acknowledgements ... 97
References ... 98
Few-layer graphene-graphene oxide composite containing nanodiamonds as metal-free catalyst in the dehydrogenation of ethylbenzene ... 101
Abstract ... 102
1. Introduction ... 103
2. Experimental ... 105
2.1. Exfoliation of expanded graphite in aqueous medium using graphene oxide as a surfactant ... 105
2.2. Self-organized decoration of nanodiamonds on the FLG-GO sheets ... 105
3. Results and discussion ... 105
4. Conclusion ... 116
Acknowledgements ... 116
References ... 117
Supporting Information ... 120
1. Synthesis of graphene oxide (GO) and GO-mediated exfoliation of expanded graphite (EG) in water ... 120
2. Self-organized decoration of nanodiamonds on the surface of GO-FLG ... 122
Reference ... 127
CHAPTER 4 ... 129
Nanodiamonds decorated graphene-carbon nanofibers 3D architecture as a metal-Free catalyst for styrene production ... 131
Graphical abstract ... 132
Abstract ... 132
1. Introduction ... 133
2.1 Few-layer graphene synthesis and characterization ... 134
2.2 Synthesis of FLG supported Ni nanoparticles ... 135
2.3 Synthesis of a 3D CNF/FLG composite ... 135
2.4 Deposition of nanodiamonds onto a CNF-FLG composite ... 135
3. Results and discussion ... 136
3.1 FLG supported Ni nanoparticles characteristics ... 136
3.2 CNF/FLG composite characteristics ... 137
3.3 ND/FLG-CNFs composite ... 138
3.4 Dehydrogenation of ethylbenzene into styrene ... 143
4. Conclusion ... 146
Acknowledgements ... 147
References ... 148
Supporting Information ... 151
CHAPTER 5 ... 155
Nanodiamonds/β-SiC Composite as an Efficient Metal-Free Catalyst for the Steam-Free Dehydrogenation of Ethylbenzene to Styrene ... 157
Graphical abstract ... 158
Abstract ... 158
1. Introduction ... 159
2. Experimental Section ... 160
2.1 Metal-free catalysts preparation ... 160
2.2 Characterization techniques... 161
2.3 Catalytic activity measurements ... 162
3. Results and discussion ... 163
4. Conclusions ... 179
Acknowledgements ... 179
References ... 180
Supporting Information ... 182
CHAPTER 6 ... 185
Green chemical route to produce hierarchical carbon nanotubes coated with nitrogen-doped porous carbon as metal-free catalyst ... 187
Graphical abstract ... 188
Abstract ... 188
2. Results and discussion ... 190
2.1. Oxygen reduction reaction (ORR) ... 194
2.2. Steam-Free Dehydrogenation of Ethylbenzene into Styrene. ... 196
3. Conclusions ... 198
Acknowledgment ... 199
References ... 200
Supporting information ... 203
Synthesis of nitrogen-doped carbon foam composite ... 203
A Highly N-Doped Carbon Phase “Dressing” of Macroscopic Supports for Catalytic Applications ... 213
Graphical and textual abstract ... 214
Abstract ... 214
1. Introduction ... 215
2. Results and discussion ... 218
3. Conclusions ... 219 Acknowledgments ... 220 References ... 220 Supporting information ... 222 Methods ... 222 References ... 229
Nitrogen-enriched carbon nanospheres decorated silicon carbide as a superior metal-free catalyst for styrene production ... 230
Abstract ... 231
1. Introduction ... 232
2. Experimental ... 234
2.1 Catalyst preparation ... 234
3. Results and discussion ... 235
3.1Morphology analyses of typical nitrogen-enriched carbon nanospheres ... 235
3.3 Physicochemical analyses of ND and ND@NMC composites ... 238
3.4 Catalytic performance on ND and ND@NMC composites ... 240
3.5 Characterization and catalytic reaction of macroscopic shaped ND@NMC/SiC composite . 242 3.6 Macroscopic ND@NMC/SiC catalyst under severe dehydrogenation conditions ... 246
4. Conclusion ... 249
References ... 250 Supporting Information ... 252 CHAPTER 7 ... 255 Summary ... 256 General conclusions ... 256 Perspectives ... 259 References ... 261 Annex……….. 264
A l’heure actuelle, la catalyse est présente dans plus de 90 % des procédés chimiques industriels et 13 des 20 produits chimiques les plus importants sont fabriqués par voie catalytique: la vente de catalyseurs représente 3 milliards d’euros par an et celle des produits qui en sont issus 300 milliards d’euros par an, soit 100 fois plus.1 Cette expansion sera poursuivie compte tenu de la forte demande dans les pays industriels mais aussi dans les pays émergents tels que la Chine, l’Inde et le Brésil, sans parler des pays du Sud-est asiatique où de fortes croissances sont à prévoir dans les années à venir.
La production de styrène a connu une croissance exponentielle depuis le premier procédé industriel dans les années 302,3, avec une estimation avoisinant les 25 millions de
tonnes par an.4 Le styrène est principalement utilisé dans la fabrication du polystyrène, des
résines acrylonitrile-butadiène-styrène (ABS) mais aussi dans la formulation de divers autres polymères, et représente aujourd’hui le ciment de la révolution des matières plastiques. L’industrie du styrène est très diversifiée, allant des produits d’emballage et de construction, jusqu'à la filière automobile. Elle capitalise ainsi un chiffre d’affaire estimé à 60 milliards de dollars par an.
Le styrène est essentiellement fabriqué par la réaction de déshydrogénation catalytique de l’éthylbenzène en styrène, procédé de fabrication faisant partie des dix procédés catalytiques les plus importants au monde.5 Les catalyseurs actuellement utilisés sont
constitués d’oxyde de fer promu au potassium et stabilisé avec de l’alumine (K-Al-Fe).6 Cependant, ils sont peu actifs et sélectifs et se désactivent rapidement au cours de la réaction par dépôt de résidus carbonés (ou coke, formé par polymérisation des intermédiaires réactionnels et notamment le styrène). Afin de réduire ce dépôt, une grande quantité de vapeur d’eau (5-15 %) est co-injectée dans le réacteur avec l’éthylbenzène, mais la génération de cette vapeur d’eau est un procédé très coûteux énergétiquement (1.5 × 109 cal/ tonne de styrène).7 De plus, de grandes quantités d’eau polluée post-réaction doivent être traitées, ce
qui représente également un coût énergétique et environnemental non négligeable pour le procédé. Il s’avère dès lors intéressant de trouver d’autres types de catalyseurs plus robustes et plus performants permettant de réaliser la transformation de l’éthylbenzène en styrène avec une conversion et une sélectivité élevée, et une meilleure stabilité en fonction du temps sous flux en l’absence de vapeur d’eau.
De nombreux travaux reportés dans la littérature ont montré que les matériaux carbonés tels que graphène, nanotubes ou nanofibres de carbone, et nanodiamants8,9,10,11 occupent un
intérêt particulier dans le milieu de la recherche et de l’industrie, en raison de leurs propriétés physiques et chimiques exceptionnelles à savoir, surface spécifique élevée et porosité ouverte, ainsi qu’une excellente inertie chimique. Ces atouts leur confèrent une place privilégiée dans plusieurs domaines d'applications tels que l’électronique,12 la conversion et le stockage
d’énergie,13 la nanomédecine,14 le traitement des eaux usées,15et surtout dans les procédés de la catalyse moderne où ils peuvent être utilisés directement comme catalyseurs ou supports de catalyseurs.16 De plus, leur dopage par d'autres éléments chimiques (N, B,...) permet de
modifier leurs propriétés intrinsèques, et par conséquent, d'améliorer leurs performances en termes d'activité et de sélectivité dans les procédés catalytiques allant de l'oxydation à l'hydrogénation, en passant par la déshydrogénation sélective. De nombreux travaux ont ainsi reportés des performances catalytiques souvent supérieures de ces catalyseurs « sans métaux » à base de nanomatériaux carbonés comparés à celles obtenues sur les catalyseurs traditionnels supportés à base de métaux ou d'oxydes de métaux, et ce, avec une très grande stabilité.17
C'est pourquoi ces catalyseurs "sans métaux" à base de carbone se présentaient comme des candidats incontournables pour remplacer le catalyseur traditionnel à base de fer dans le procédé de déshydrogénation directe de l’éthylbenzène en styrène.
D'autre part il s'avère que les catalyseurs carbonés sans métaux à base de nanodiamants présentaient de loin les meilleures performances catalytiques, en termes de conversion et sélectivité dans la réaction de déshydrogénation (DH) de l'éthylbenzène en styrène. La forte activité catalytique des nanodiamants a été attribuée à la présence de deux états d’hybridation de carbone, sp2 et sp3, sur la surface du matériau et qui lui confère des propriétés d’adsorption remarquables, bases de la catalyse hétérogène.
L'objectif de la thèse porte sur l'élaboration de catalyseurs "sans métaux" à base de carbone, et plus essentiellement à base de nanodiamants, pour le procédé de déshydrogénation directe (DH) de l'éthylbenzène (EB) en styrène (ST) en absence de vapeur d’eau.
Nous avons dans un premier temps, développer puis utiliser des matériaux à base de carbone tels que les nanotubes (CNTs) ou nanofibres de carbones (CNFs), le graphène multi-feuillets (FLG), ou les nanodiamants (NDs), et les tests catalytiques ainsi réalisés ont révélés une nette supériorité des performances catalytiques du catalyseur à base de nanodiamants comparé aux autres allotropies de carbone et au catalyseur industriel à base de fer (Figure 1). Ces performances sont essentiellement dues à la concentration des défauts de structures présentes en surface de ce matériau et au rapport surface/volume très élevé. De plus, comme on peut le constater, en l’absence d’eau dans la charge, le catalyseur industriel se désactive très rapidement pour atteindre un palier plus bas. Le catalyseur à base de nanodiamants présente également une sélectivité en styrène légèrement supérieure à celle du catalyseur industriel.
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0 5 10 15 Time on stream (h) ND ST rate ND ST Sel. ST se lec ti v it y ( %) 0 20 40 60 80 100 K-Fe STrate ST rate ( mmolg -1 cat h -1 ) K-Fe ST sel.Figure 1 : Comparaison des performances catalytiques obtenues sur nanodiamants et sur catalyseur industriel à base de fer promus au potassium (Conditions de test: T = 600°C, P = atmosphérique, [EB] = 2,8 vol. %).
Néanmoins les investigations en microscopie réalisées sur les nanodiamants ont montré une distribution de taille de particules comprise entre 3-10 nm, et une forte tendance à s’agglomérer, ce qui en catalyse pourrait se traduire par une diminution de la densité des sites actifs et une activité qui pourrait être largement sous-estimée. Dans l’optique d’améliorer l’activité catalytique des nanodiamants, nous avons mis en œuvre un protocole permettant de synthétiser des matériaux hybrides consistant à disperser des particules de nanodiamants sur des supports à base de graphène (FLG) et graphène oxyde (GO). Les particules de diamant ont aussi été dispersées sur des matériaux hybrides carbonés avec une structure hiérarchisée constituée par une couche de nanofibres de carbone (CNFs) sur la surface du graphène. L’ajout de cette troisième dimension permet de mieux ancrer les nanodiamants, grâce à la présence de plans prismatiques sur la surface des nanofibres de carbone (ND/CNF-FLG), et de disperser des nanodiamants générant ainsi des sites actifs avec une meilleure accessibilité aux réactifs, ce qui permet l’obtention d’une excellente activité et une meilleure évacuation des produits de réaction hors de la zone catalytique, favorisant ainsi une bonne sélectivité et une formation plus faible de coke. Ces matériaux hybrides ainsi synthétisés ont accusé des performances catalytiques relativement élevées avec une meilleure activité spécifique du catalyseur 3D ND/CNF-FLG équivalent à 11,5 mmolST gND-1 h-1 soit plus de quatre fois supérieure à celle des nanodiamants non supportés, et dix fois supérieure à celle du catalyseur
industriel à base de fer. Le principal avantage du catalyseur 3D ND/CNF-FLG comparé au catalyseur 2D ND/FLG réside dans sa très grande stabilité obtenue dans des conditions de test extrêmes reproduisant celles utilisées dans les unités industrielles (Figure 2), à savoir, hautes températures, pression partielle de l’éthylbenzène élevée, test longue durée.
0 10 20 30 40 50 60 70 80 0 5 10 15 20 25 30 35 600°C 575°C 550°C Time on stream (h) ST . ra te ( mmolg -1 ND h -1 ) 0 20 40 60 80 100 120 140 0 5 10 15 20 25 30 35 600°C 575°C 550°C Time on stream (h) ST . ra te ( mmolg -1 ND h -1 ) A B
Figure 2. Influence de la température de réaction sur les performances catalytiques obtenues sur catalyseurs (A) ND/CNF-FLG et (B) 2D ND/FLG. (conditions opératoires: masse de catalyseur = 300
mg, 2.8 % d'EB dans l’hélium, flux total de gaz = 30 mL·min-1).
Cependant, il est à noter que les nano-carbones sont généralement obtenus sous forme d’une fine poudre nanoscopique qui rend leur manipulation et leur transport difficiles. Dans le domaine de la catalyse hétérogène, leur utilisation est fortement entravé dans certaines applications fonctionnant avec des réacteurs a lit fixe, car limitées par des phénomènes liés à la perte de charge18 à travers le lit catalytique, et dans le cas des lit agités, la récupération du catalyseur après réaction est souvent très difficile. Face à ces contraintes il était nécessaire de développer une nouvelle génération de catalyseurs avec une structure macroscopique permettant ainsi de répondre aux nouvelles exigences des procédés industriels modernes de catalyse, à savoir haute pression partielle et vitesse de gaz élevée. Nos recherches se sont donc orientées sur l’utilisation de supports de catalyseurs tels que l’alumine ou la silice couramment utilisés dans les procédés de catalyse hétérogène. Néanmoins, les performances catalytiques ne sont pas à la hauteur de notre attente. Un nouveau support de catalyseurs, le carbure de silicium (b-SiC), a été également choisi pour disperser les nanodiamants en raison de sa bonne conductivité thermique, sa résistance mécanique élevée, son inertie chimique et son excellente résistance à l’oxydation et à la corrosion.
La fabrication du SiC sous sa forme b est actuellement développée et industrialisée par la société SICAT Sarl (www.sicatcatalyst.com), et plusieurs mises en forme (grains, extrudés, pellets, mousse alvéolaire) sont disponibles selon le type d’applications visées (Figure 3).
Poudre Extrudé Mousse alvéolaire
Figure 3. Différentes formes à base de carbure de silicium de structure b utilisées pour la dispersion des nanodiamants.
Nous avons réussi à synthétiser en une seule étape des catalyseurs hybrides sous forme macroscopique, à base de nanoparticules de NDs décorées sur différentes formes de carbure de silicium, e.g. poudre (40 µm), grains (250-425 µm) et mousse alvéolaire (plusieurs centimètres de dimension), afin de voir l’influence de la morphologie du support sur l’activité du catalyseur. Les tests catalytiques ont montré une meilleure activité du catalyseur à base de NDs supportés sur mousse alvéolaire (ND/SiC foam) par rapport aux grains ou à la poudre (Figure 4), ce qui est en parfaite corrélation avec les données de la Figure 2B, illustrant les mesures de perte de charge réalisées sur différentes formes de SiC données. Le catalyseur ND/SiC(foam) possède aussi une grande surface et une porosité ouverte constituée essentiellement d’un vaste réseau mésoporeux et macroporeux. La présence de ponts reliant la structure permet de créer des microturbulences au sein du mélange réactionnel favorisant ainsi le contact effectif entre les réactifs et les sites actifs et par conséquent une meilleure réactivité. L’activité spécifique du ND/SiC(foam) est de 12,86 mmolST gND1h-1, soit quatre fois supérieure à celle des NDs non supportés ,et 12 fois celle du catalyseur traditionnel K-Fe.
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ND13%.wt-SiC(foam)Time on stream (h)
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ND13%.wt-SiC(grains) ND13%.wt-SiC(grains) ND13%.wt-SiC(powde) ND13%.wt-SiC(powder) 0 2 4 6 8 10 12 14 0 1 2 3 4 Foam Sphere (2-4 mm) Extrudate (1-2 mm) Grain (0.4mm) Powder (<0.15 mm) ug (m/s) ∆ P /L ( h 1 0 5 P a /m )A
B
Figure 4. (A) Comparaison des performances catalytiques de catalyseurs à base de nanodiamants supporté sur différentes formes de SiC. (B) Mesures des chutes de pression obtenues sur différentes morphologies de SiC
La Figure 5 illustre les différentes grandes étapes (dispersion, macronisation, dopage) pour synthétiser les différents matériaux carbonés qui seront utilisés ensuite comme catalyseurs sans métaux dans la réaction de déshydrogénation directe de l'éthylbenzène en styrène.
Figure 5. Illustration d’une série de matériaux carbonés synthétisés durant la thése, utilisés comme catalyseurs sans métaux dans la réaction de déshydrogénation de l'éthylbenzène en styrène.
Quelques images représentatives obtenues par microscopie électronique à balayage et en transmission sont présentées sur la Figure 6 pour illustrer les résultats discutés ci-dessus. Nous pouvons observer sur les clichés B, C, F et G une nette amélioration de la dispersion des nanodiamants supportés sur FLG ou sur SiC.
A
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20nm G 1µm 1µm 20nmD
2 mm Φ= 1-2 mm 1mmH
β-SiC NDs 20nmE
20µmI
Disordered C-N composite CNT 10nmA
B
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Figure 6 : Images obtenues par MEB et MET des catalyseurs sans métaux à base de nanodiamants (A, E) Nanodiamants, (B, F), Nanodiamants sur FLG, (C, H) Nanodiamants sur mousse de SiC et (E, I) Mousse de CNTs recouverte par une couche de carbone mésoporeux dopé avec de l’azote.
Différentes méthodes plus ou moins simples de synthèse de ces matériaux ont été utilisées (Figure 7): la technique de dépôt chimique en phase gaz (CVD)19 notamment pour la
synthèse des CNTs et CNFs et consistant à envoyer un mélange gazeux contenant la source de carbone (exemple C2H6/H2) sur un catalyseur de croissance à base de Fer ou Nickel, le broyage mécanique dans le cas des mousses de CNTs, l’imprégnation directe et la sonication pour disperser les phases actives comme les nanodiamants.
Les matériaux ainsi synthétisés ont été soumis à différents traitements thermiques afin d’éliminer les éventuelles impuretés et de contrôler des paramètres physico-chimiques tels que la porosité et les espèces azotés présents à la surface (dans le cas du dopage à l'azote), ou le degré de graphitisation du matériau carboné, permettant ainsi d'obtenir une meilleure stabilité du catalyseur selon le type de test catalytique souhaité.
+NDs FLG CNF-FLG CNF-FLG +Ni (CVD) +NDs ND/FLG ND/CNF-FLG Sonication SiC ND/SiC CNT Dextrose Carbonate d’ammonium Acide citrique Mousse de N-CNT Mecanical mixing usse de N-NDs N-NDs +NDs +Nitrogen source Wet impregnation +NDs
Figure 7 : Méthodes de synthèse des matériaux carbonés développés au cours de la thèse : dépôt par sonication, dépôt par imprégnation et dépôt par mélange mécanique.
Les résultats catalytiques obtenus sur ces catalyseurs sans métaux ont permis de montrer des performances catalytiques largement supérieures à celles du catalyseur industriel à base d'oxyde de fer (Figure 5), et s'avèrent être d'une très grande stabilité en fonction du temps de réaction, ce qui laisse entrevoir un possible développement de l'application à l’échelle industrielle.
Toujours dans cette même optique de développer un catalyseur performant sous une forme macroscopique, d’autres paramètres tels que le dopage ont aussi été étudiés durant cette thèse. Durant cette dernière décennie les matériaux à base de carbone 1D et 2D dopés ont suscité un intérêt croissant tant dans le milieu de la recherche que dans celui de l’industrie, en raison de leur capacité à promouvoir plusieurs procédés catalytiques très demandés, e.g. la réaction électrochimique de réduction de l’oxygène,20,21 ou l’oxydation sélective d’impuretés pour la purification des gaz.22
Le dopage a l’azote est souvent réalisé dans des conditions de températures très élevées (650-800°C), ce qui rend le procédé peut rentable et les précurseurs d’azote utilisés sont souvent très toxiques, i.e. pyridine,23 aniline,24 melamine,25polypyrrole,26 ou ammoniac27,28. Il
est à noter également que pour les précurseurs de carbone on fait appel à des hydrocarbures qui présentent des problèmes d’inflammabilité non négligeables.
Face à ces contraintes nous avons développé un composite à base de nanotubes de carbone dopés par une méthode simple et à moindre coût. De plus, elle présente l'avantage d'utiliser des produits alimentaires donc plus écologiques et économiques, à savoir le dextrose (source de carbone), l’acide citrique (agent polymérisant) et le carbonate d’ammonium (source d’azote et agent porogène). Ce composite (mousse de N@CNT), synthétisé à basse température (170°C) et formé d’un enchevêtrement très dense de nanotubes de carbone, synthétisés par voie CVD, est constitué de méso et macrospores (Figure 8). Les mousses de N@CNT se présentent sous forme de mousse de dimension allant de 1 mm à plusieurs centimètres, et donc plus intéressants pour d’éventuelles applications industrielles. Les mousses de N@CNT présentent aussi des avantages non négligeables pour des applications en catalyse: grande surface effective, meilleure accessibilité aux réactifs, mise en forme contrôlée et une porosité ouverte adaptable en fonction des réactions visées, propriétés mécaniques intéressantes et enfin, une récupération facile du catalyseur dans les réactions en phase liquide.
Figure 8. Image MEB (a,b) N@CNT foam après synthèse a basse température, (c,d) N@CNT
obtenu après traitement thermique à 450°C sous air et (d,e) illustration des mises en forme possibles et de la résistance mécanique du composite
Ce type de catalyseur a montré des performances très prometteuses dans le domaine des piles à combustible où il témoigne d'une activité très similaire à celle du catalyseur commercial à base de platine supporté sur carbone (Pt-20 %/C), mais avec une désactivation moindre pour des tests longue durée. Dans la réaction de déshydrogénation de l’éthylbenzène en styrène la mousse de N@CNT présente une meilleure activité et stabilité comparée non seulement au catalyseur industriel mais aussi aux nanodiamants supportés.
0
20
40
60
80
100
0
4
8
12
16
20
13 wt% NMC-ND/SiC(F) 16 wt% ND/FLG(P) 13 wt% ND/SiO2(P)Catalyst
ST
yie
ld
(mm
ol
S Tg
N D -1h
-1)
CNTs K-Fe FLG SiC 44 wt% ND/SiC(F) 33 wt% ND/SiC(F) 13 wt% ND/SiC(F) NDsStyrene yield
ST
sel
.
(%)
Styrene sel.
Unsupported catalysts Supported catalysts
Figure 9. Comparaison des catalyseurs synthétisés à base de carbone (supportés ou non) avec le catalyseur industriel K-Fe. (Température de réaction 550°C, P = atm, [EB] = 2,8 vol.%)
La Figure 9 regroupe les performances catalytiques des divers catalyseurs « sans métaux » testés dans le cadre de cette thèse ainsi que celles des supports seuls et aussi du catalyseur industriel à base de fer. Mais le fait le plus marquant de la thèse est que nous avons réussi à développer un catalyseur carboné sans métaux (NMC-ND/SiC) et extrêmement actif à partir de produits alimentaires, à savoir le sucre, le citron et la levure (dextrose, acide citrique et carbonate d’ammonium respectivement). Ce matériau facile à synthétiser et à coût très modéré, innovant et révolutionnaire s’est avéré non seulement plus performant dans la réaction de déshydrogénation directe de l’éthylbenzène en styrène que le catalyseur industriel à base de fer, mais aussi plus performant que ses homologues à base de carbone. De plus, ce catalyseur présente aussi de meilleures performances que les catalyseurs traditionnels dans d’autres domaines d'applications très demandés, tels que le traitement des eaux, la détection de pesticides, la réduction de l’oxygène dans les piles combustibles ou l’oxydation partielle des traces d’H2S en soufre élémentaire.
En conclusion, nous avons développé dans le cadre de la thèse une nouvelle famille de catalyseurs sans métaux à base de carbone supportés sur diverses structures hôtes macroscopiques pour le procédé de déshydrogénation sélective de l’éthylbenzène en styrène en absence de la vapeur d’eau. Les catalyseurs ainsi développés sont extrêmement stables
pour le procédé visé, permettant ainsi de réduire d’une manière significative les coûts énergétiques du procédé traditionnel où de larges quantités de vapeur sont co-injectées avec le réactif, mais aussi son impact sur l’environnement. Dans la suite de la thèse un nouveau procédé de synthèse des composites carbonés dopés avec de l’azote a également permis de synthétiser de nouveaux catalyseurs sans métaux extrêmement actifs et sélectifs pour le procédé testé dans la thèse, mais aussi dans plusieurs autres procédés tels que le traitement des eaux, la réduction de l’oxygène dans les piles à combustibles, l’oxydation sélective du sulfure d’hydrogène en soufre élémentaire, la détection des pesticides où des tests à grande échelle ont déjà été réalisés sur plusieurs localités en Alsace.
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World population is facing an unprecedented growth since the last decades and alongside with this phenomenon, goods demand also following the same trend and even more for the future. On the same time, the amount of energy required for producing goods should be reduced as much as possible in order to cope with the general policy regarding the problems linked with the greenhouse gas.1
Catalysis is contributed to more than 80 % of the whole chemical processes for producing day life products.2 Catalysts allow one to reduce the energy and waste products in
each chemical conversion step which is in line with the more stringent legislation dealing with the global climate change linked to an increasing energy demand worldwide. Nowadays, most of the catalytic systems are constituted by an active phase, i.e. metals, oxides, alloys, sulfides, carbides, etc,3,4,5,6,7 deposited on an appropriate support. The support provides a high surface
area to efficiently disperse the active phase in order to maximize the catalytic effective surface area for the reaction.8 The support porosity is also finely tuned in order to avoid as
much as possible micropores as this later will lead to an apparent higher residence time of the intermediate product, and thus, favoring the formation of by-products which contribute to the lowering of the overall selectivity of the process. The support also prevents catalytic activity lost during the course of the reaction through the active phase sintering or encapsulation by side-products. However, deactivation through sintering and catalytic surface poisoning or encapsulation always occur leading to a gradual catalytic deactivation. In order to maintain the catalytic performance the spent catalyst is submitted to a periodical oxidative regeneration which allows the removal of deposit residues on the active phase surface or in the catalyst pore size.9 The repeated regeneration could induce slow deterioration of the catalyst
performance which calls for complete replacement of the catalyst. Therefore, it is of interest to develop new catalytic systems which exhibit higher resistance towards sintering and encapsulation through carbonaceous residue deposit which allow one to avoid frequent oxidative regeneration which is cost incentive for the process.
Styrene (ST), also known as phenylethylene, vinylbenzene, styrol, or cinnamene, C6H5– CH=CH2, is one of the most important unsaturated aromatic monomers in modern petrochemical industry. It occurs naturally in small quantities in some plants and foods. In the nineteenth century, styrene was isolated by distillation of the natural balsam storax.10 It
has been identified in cinnamon, coffee beans, peanuts, and it is also found in coal tar.
The world production of styrene monomer is significantly increased since the last decade to reach about 26 Mt per annum in 2012 and continue to face a high growing rate in the future (Fig. 10).11 The main licensors of styrene production processes are Badger,
Lummus, LyondellBasell, Shell, DOW and BASF. The annual production of styrene in the U.S.A. already exceeds 6 Mt and continuous growing.78
Figure 10. World styrene supply and demand during the last few years.11
The styrene commercial processes were developed in the 1930s independently and simultaneously by BASF in Germany (Figs. 12 and 13) and by DOW Chemical in the USA.12,13 The need for synthetic styrene – butadiene rubber (Styrene – Butadiene Rubber (SBR)) during World War II provided the impetus for large-scale production.
Figure 12. Reaction kettles in the BASF polystyrene production plant.13
Styrene is the raw material involving in the production of various polymeric materials, the most important being polystyrene, synthetic rubbers, plastics, acrylonitrile–butadiene–
styrene (ABS) and styrene-acrylonitrile (SAN) resins, latex (SBL). The international styrene industry is a diversified capitalizing approximately USD 60 billion (annually) comprising thousands of companies, facilities, and employees throughout the world. Styrene owns its popularity to the easily reacting double bond that can undergo polymerization reactions with themselves or with other monomers.
Styrene Reaction Temperature 100°C 150°C 200°C Polystyrene
Figure 13. Schematic of BASF's early tower process for the continuous polymerization of styrene. This configuration was designed by Wulff and Dorrer in the early 1930s. Polymerization was
thermally initiated and the exothermicity was controlled by heat transfer tubes.13
Manufacturers use styrene-based resins to produce a wide variety of everyday goods ranging from cups and utensils to furniture, bathroom, and kitchen appliances, hospital and school supplies, boats, sports and recreational equipment, consumer electronics, automobile parts, and durable lightweight packaging of all kinds (Fig. 14A). It provides essential raw materials and products for nearly all major European, American, and Asian industries, from automobiles and construction to electronics and packaging (Fig. 14B), just to cite a few.
Figure. 14. Examples of day-life products produced from styrene.
Currently styrene is industrially produced mainly by two catalytic processes. In the first one, which is a modification of the Halcon process, styrene is obtained as a by-product in the epoxidation of propene with ethylbenzene hydroperoxide over Mo complex-based catalysts. This process is commercialized by ARCO Chemical (formerly Oxirane) and by Shell. About 2.5 kg styrene is obtained per kilogram of propylene oxide. Approximately 1.2×106 t/year of styrene are currently produced with this technology.11 The second one and also the most
A
important manufacturing route to styrene is the direct dehydrogenation (DH) of ethylbenzene (EB), which accounts for more than 90% of the worldwide capacity production.14
Under ordinary conditions, ethylbenzene is a clear, colorless liquid with a characteristic aromatic odor. Ethylbenzene is an irritant to the skin and eyes and is moderately toxic by ingestion, inhalation, and skin adsorption. The main physical characteristics of ethylbenzene are summarized in Table 1.
Table 1. Physical characteristics of ethylbenzene.
M(C6H5-CH2-CH3) 106.17 Flash point, Tag Closed Cup 15°C Boiling point (101 kPa) 136.19°C Autoignition point 460°C
mp -94.9°C Refractive index, nD (20°C) 1.49588
Heat of vaporization (ΔHy) 335 J/g Density, g/mL (20°C) 0.8669
Styrene is a colorless liquid with a distinctive, sweetish odor. Styrene is miscible with most organic solvents in any ratio. It is a good solvent for synthetic rubber, polystyrene, and other non- cross-linked high polymers. Styrene and water are sparingly soluble in each other. Some physical properties of styrene are summarized in Table 2. Vapor pressure is a key property in the design of styrene distillation equipment.
Table 2. Physical characteristics of styrene.
M(C6H5-CH2=CH2) 145.15 Flash point, Tag Closed Cup 31.1°C Boiling point (101 kPa) 145.15°C Autoignition point 490°C
mp -30.6°C Refractive index, nD (20°C) 1.5468
Heat of vaporization (ΔHy) 421.7 J/g Density, g/mL (20°C) 0.9050 Flammable limits in air 1.1-6.1 % Heat of polymerization (ΔHp)
(20°C)
-69.8 kJ/mol
The industrial catalyst is the iron-based catalyst promoted with potassium and different foreign elements, in order to increase the lifetime of the catalysts, has been extensively used for styrene production.15 The iron promoted potassium catalyst was constituted by about 84
wt. % of iron oxide Fe2O3 (hematite) and at least 13 wt. % of potassium oxide, small amounts of alumina (Al2O3) and chromium (Cr2O3) which acting as structural promoters to increase the lifetime of the catalysts. Oxides such as MgO, Cr2O3, CeO3 and MoO3 have also been incorporated inside the iron-based catalyst to improve the dehydrogenation selectivity, but have virtually no effect toward the dehydrogenation activity. The role of the promoter, i.e. potassium, is to increase the DH activity by more than one order of magnitude compared to the DH activity of iron oxide, and also to increases the selectivity to styrene as well as the stability of the catalyst under operation.
Figure 15. Adiabatic dehydrogenation of ethylbenzene (EB). a) Steam superheater, b) Reactor, c)
High-pressure Steam, d) Low-pressure Steam, e) Condenser, f) Heat exchanger.16
The process can be run industrially either adiabatically or isothermally over a fixed bed reactor in which the reactants are passed over the catalyst bed employing radial or axial flow. Over 75 % of all operating styrene plants carry out the dehydrogenation reaction adiabatically17 in multiple reactors or reactor beds operated in series (Fig. 15).The heat
necessary to the reaction is provided at the inlet to each stage, either by injection of super-heated steam or by indirect heat transfer. The inlet temperature of the feed is about 910 K and the molar ratio of steam to EB is varies from 7 to 12, depending on the catalyst formulation and type of the used process. EB conversion in the first reactor is about 35 mol% to give a total conversion of 65 mol% for the process. This process is licensed by Badger, ARCO and Shell etc.
Isothermal dehydrogenation process (Fig. 16) was pioneered by BASF and has been used by them for many years. The reactor is built like a shell-and-tube heat exchanger, heated indirectly by the hot flue gas. Vaporized ethylbenzene is mixed with steam (steam/EB molar ratio of 3 - 6) and passed into the tubular reactor (600°C) filled with suitable catalyst.
Figure 16. Isothermal dehydrogenation of ethylbenzene (EB). a) Heater, b) Steam superheater, c)
Reactor, d) Heat exchanger, e) Condenser.16
This reaction proceeds thermally with low yield and catalytically with high yield. As it is a reversible gas- phase reaction producing 2 mol of product from 1 mol of starting material, low pressure favors the forward reaction. It is mostly established that one of the most important role of potassium promoter consist of selectively turn the catalyst toward the formation of an active phase KFeO2.18,19,20 Hirano14,21 was the first who identified by XRD and XPS analysis that the KFeO2 are the active phase, and then work developed by Muhler22 demonstrated by a revolutionary model that the active state is resulted from an equilibrium between KFeO2 and K2Fe22O32. The active phase can be reduced by hydrogen to KOH and Fe3O4 (magnetite), leading to an irreversible deactivation. The schematic life cycle of the traditional potassium-promoted iron-based catalyst is illustrated in Figure 17.
Carbon deposition K2Fe22O34+ Fe2O3 KFeO2 Fe2O3 Fe3O2 H2O/EB KFeO2 K2Fe22O34 C + H2O CO + H2 CO + H2O CO2+ H2 KFeO2+ K2Fe22O34 KOH, Fe3O4 Fe3O4+ segregated promotor phases Fe3O4 Cor e Shel l Regeneration with H2O Spatial disintegration Inactive state Inactive state Formation Formation Deactivation H2 -K+ +K+ +K+ -K+
Figure 17. Schematic life cycle of the iron-based catalyst with potassium without any promotor
additives.22
The C-H groups of the EB-ethyl group get deprotonation at basic oxygen sites and two hydroxyl groups are formed at the surface of the catalyst.23 The active state of the Fe–K
catalyst was analyzed by transient response experiments and it has been reported that initially high activity was correlated with Fe3+ state and the reduction from Fe3+ to Fe2+ state caused deactivation of the catalytic activity.24 Simultaneously or subsequently an electron transfer to
acidic Fe3+ sites is required before the styrene molecule can desorbs from the catalyst surface. Finally, the hydrogen atoms forming the two hydroxyl groups recombine to form hydrogen molecule which further desorb from the catalyst surface, the reduced Fe2+ species get
re-oxidation to Fe3+, and the basic oxygen sites are reestablished for the next turnover cycle. In the proposed active KFeO2 surface phase, potassium saturated Fe-O bonds increase the basicity of the oxygen sites. They also must be located in an adequate geometry with respect to the acidic Fe3+ sites, so that an effective deprotonation of the EB molecule becomes possible.
The EB dehydrogenation is an endothermic and reversible reaction with an increase in the number of molecules in agreement with the reaction mechanism. The dehydrogenation reaction was conducted at temperature at 600-700°C,25,26,27,28 high equilibrium conversion can
be achieved by a high temperature and a low EB partial pressure. The main reaction product is hydrogen and styrene (Equation 1). The selectivity of the process is extremely high (> 95%) and the main reaction products are hydrogen and styrene. However, it is worthy to note that the conversion per pass is not complete and the exit product also contains residual ethylbenzene and steam which calls for recycling. The typical by-products of the EB dehydrogenation are benzene (~1%) and toluene (~2%) (Equations 2 and 3). Benzene can be recycled to the alkylation process to make EB and toluene can be sold for a by-product credit. Hydrogen can also be removed from the product stream and sold, if it can be used in the nearby area. Otherwise, its heating value can be used for the process itself. The unreacted EB was recycled in the DH process.
Catalyst KFeO2/K2Fe22O34
+
H2 ΔH= 124.9 KJ.mol-1 (1)+
H2C CH2 ΔH= 101.8 KJ.mol-1 (2)+
CH4 ΔH= -64.5 KJ.mol-1 (3)During the course of the reaction the iron active sites were gradually deactivated by carbonaceous residues generated by the reaction leading to a drastic decrease of the DH activity. In order to maintain the active sites from these carbonaceous residues an excess of steam was co-feeded with the reactant onto the catalyst,18 which could block the most surface
iron oxide catalyst, the catalyst becomes self-cleaning (through enhancement of the reaction of carbon with steam to give carbon dioxide, which is removed in the reactor vent gas). The overall effect of the increase of the steam-to-hydrocarbon ratio is to increase the activity, the selectivity at the same level of conversion (or the conversion at the same value of selectivity), the lifetime and stability of the catalyst. It is believed that water vapor does not participate in the reaction itself, i.e. not involved in the rate determining step of the reaction mechanism, but acts purely as an oxidant agent to prevent active site encapsulation by carbonaceous residue (Equation 5) and also as inert dilution agent to lower the partial pressure of ethylbenzene, shifting the equilibrium toward styrene and minimizing the loss to thermal cracking. The steam also supplies the necessary heat to the reaction according to the endothermic character of the process. Indeed, for such endothermic reaction the catalyst temperature decreases as increasing the dehydrogenation activity, i.e. the ethylbenzene conversion per unit time, which will further lead to a decrease of the catalyst performance. Steam presence allows one to reduce such catalyst temperature lost during the course of the reaction.
+
5H2 ΔH= 1.72 KJ.mol-18C
(4)
C
+
2H2O CO2+
2H2 ΔH= -64.5 KJ.mol-1 (5)The general accepted reactions and side reactions taking place during the EB dehydrogenation other than the dehydrogenation of ethylbenzene to styrene are mentioned above (some of these reactions are thermodynamically favored at low temperature, while others are at higher temperatures),29 and phenomena which generates these processes are
Direct reactions
Ethylbenzene styrene + H2 (dehydrogenation) Ethylbenzene benzene + C2H4 (cracking) Ethylbenzene + H2 toluene + CH4 (hyrogenolysis/hydrocracking) Ethylbenzene + CO2 CO + H2 (dry reforming)
Consecutive reactions
H2+ CO2 H2O + CO (reverse water-gas-shift) Styrene precursors of coke (oligomrisation, polymerization, condensation) Precursors of coke coke + H2 (dehydrogenation) (Precursor of) coke + CO2 CO (reverse Boudouard)
Figure 18. Nature of the reactions taking place during the dehydrogenation of ethylbenzene to styrene
The problems encountered in the current ethylbenzene dehydrogenation process are summarized below:
1) The dehydrogenation of EB to styrene is equilibrium limited and highly endothermic (ΔH°298 =28.1 kcal/mol). The reversibility of the dehydrogenation process thermodynamically hinders maximum yields of styrene. The technical EB conversion is limited below 60%, to keep an acceptable high selectivity to styrene. The limited styrene yields and the low EB conversions achieved per pass through the reactor lead to the necessity of a reactant recycle. Indeed, for styrene polymerization applications, styrene has to be purified to more than 99.8%. The separation of un-reacted EB and co-products from styrene is costly due to the close boiling points, especially for EB (136°C) and styrene (145°C).
2) The high sensitive of the iron-based catalyst towards coke formation and encapsulation along with the high energy consumption due to the use of excess steam for preventing DH activity loss during the process. Indeed, the high amount of steam leads to a huge energy consumption of the process which can be estimated to be 1.5 × 109 cal/ ton of styrene30 and
also to a large amount of wastewater containing trace amount of hydrocarbons which needs to be recycled downstream. The use of steam causes also other serious limitations, namely, (i) consumption of ethylbenzene and of styrene by steam-reforming, (ii) chemical reaction of
steam with K2CO3 forming free KOH leading to the gradual loss of the promoter and finally, to the irreversible catalyst deactivation.
3) The K-Fe catalyst slowly deactivates due to surface encapsulation by coke, despite in the presence of steam, with operation and typically needs to be replaced every 1 - 2 year. In view of the scale of the reactors used, this is an expensive operation.
The number of problems that are encountered in the current commercial process for styrene production is the driving force to develop alternative technologies. It is therefore of high interest to find new catalysts which could be operated under a more environmental conditions, i.e. higher stability against deactivation by carbonaceous residue in order to reduce or eventually phase-out the need of steam in the process, while keeping similar catalytic performance. The catalyst should also display a high catalytic stability as a function of time on stream in order to reduce as much as possible replacement and solid waste generation. The catalyst needs to be robust in terms of oxidative resistance in order to withstand periodical oxidative regeneration. The catalyst production price should be also low enough for being competitive with that of the iron-based catalyst operated nowadays in the industrial process. Last but not least the catalyst should also be easy to produce in macroscopic shape for coping with the industrial parameters of operation.
Carbon-based materials, i.e. carbon black, carbon nanotubes/nanofibres, graphene and nanodiamonds31,32,33,34,35 have received an ever increasing scientific and industrial interest
during the last decade36 for several potential fields of application such as electronic,37 energy
conversion and storage,38 medicine,39 wastewater treatment40 and catalysis.41
Carbon materials, and especially nanocarbon consisting of carbon nanotubes/nanofibers, graphene and nanodiamonds (Figure 19), just to cite a few, play currently a privileged role and interest in the research and industrial environments, due to their numerous attracting properties such as high surface area and high open porosity, that can also be tailored depending to the downstream application, mechanical stiffness, chemical inertness, large ability to be chemically functionalized.42,43,44 The surface hydrophobicity/hydrophilicity of
these carbon-based materials can be also tuned as well in order to adapt the catalyst to the reaction needs. These noticeable features make carbon-based materials as a potential candidate in the field of catalysis in numerous processes, either as catalyst support or directly as catalytic material itself .45,46,47 Carbon-based materials have witnessed a large scientific and
synthesis and in-depth characterization, involving both experimental and theoretical studies, which allow one to set rational bases for their subsequence development. The large possibility of doping the carbon-based materials with numerous foreign elements also opens a new area for potential catalytic applications reaching far beyond the fields under development today. Taken into account the large number of catalytic processes that involve nanocarbons, we will restrict the discussion here to the field of oxidative dehydrogenation and direct dehydrogenation processes. For the synthesis and other applications one can find several update reviews devoted to such area.48,49,50,51,52
The recent reports published by different research groups have evidenced that nanocarbon-based materials can be efficiently employed as metal-free catalysts with high performance and stability in both oxidative and direct dehydrogenation processes for the styrene production in place of the industrial iron-based ones.
a
b
c
e
d
Figure 19. Crystal structures of different allotropes of carbon. (a) nanodiamond (0D), (b) graphite (3D), (c) graphene (2D), (d) nanotubes (1D), and (e) buckyballs (0D)
Indeed, alternative catalytic routes involved carbon-based metal-free catalysts have been extensively studied during the last decades deal with the direct dehydrogenation (DH) or oxidative dehydrogenation (ODH) processes.53,54 Numerous articles have pointed out that the
oxidative dehydrogenation (ODH), using carbon allotropes (Fig. 19), is an elegant and very promising alternative to conventional dehydrogenation process where deactivation represents
the main drawback. The metal-free carbon-based catalysts exhibit a relatively high DH or ODH performance compared to the traditional catalysts along with an extremely high stability as a function of time on stream thanks to their high resistance towards coke encapsulation. The high catalytic performance could be directly attributed to the high effective surface area of these nanocarbon-based catalysts which providing higher contact surface between the active sites and the reactant. The nanoscopic dispersion of the active phase present on such carbon-based catalysts also plays a vital role for the catalytic process as well.
However, aside from the fundamental point of view these nanocarbons should be produced in a macroscopic shape in view of the future industrial development. Indeed, nanocarbons are present in a powder form and thus, generate a large pressure drop across the catalyst bed which could significantly modify the activity and selectivity of the process (Figure 20). The macronization can be performed either by self-assembly of the carbon materials itself or by decorating them on macrocopic support such as alumina or silicon carbide, examples of these materials are presented in Figure 21.
Figure 20. Pressure drop measurements on the various macroscopic composites with different shapes.
The nanoscopic shape also generates problems linked with the handling and transport as well as those linked with the charging and discharging of the catalyst bed as well. Figure 21 summarizes the different dimensions of these nanocarbons including the supported form for
industrial applications. It is worthy to note that the macroscopic shaping not only reduce the problem of pressure drop and the dispersion of the active phase, but also allows one to control the problem of heat and mass transfer during the reaction by adapting the macroscopic shape to the reaction conditions. Previous works on the silicon carbide containing cobalt catalysts have shown that the use of a thermal conductive support greatly improve the liquid hydrocarbons selectivity in the Fischer-Tropsch synthesis process by favoring the heat transfer within the catalyst body.55 Similar results have also been reported on the beneficial
effect of the thermal conductive SiC support on the endothermic dehydrogenation of n-butane reaction.56
Figure 21. Various carbon-based materials, from nanoscopique to 3D macroscopic shape.
The exothermic ODH process to convert ethylbenzene (EB) to styrene (ST) operates in the presence of oxygen to allow the removal of the formed hydrogen as steam. On the other hand, the strongly exothermic ODH of hydrocarbons also reduces the problem linked with the catalyst temperature lost as encountered with the endothermic DH process. The presence of oxygen also allows operating the process at lower reaction temperature, 400-500°C, compared
to that operated in the DH process. The presence of oxygen and steam (generated in-situ during the course of the reaction) also contribute to the reducing of the amount of cracking products and also the coke formation and thus, the use of extra steam can be avoided which leads to a significant energy saving for the process. In addition, the ODH process also eliminates most limitations imposed by thermodynamic compared to the DH process and potentially higher yields (90%) can be obtained per pass. However, it is worthy to note that the ODH is an exothermic process and thus, reaction conditions need to be controlled in order to reduce as much as possible the direct oxidation of the starting compounds through CO/CO2 formation which significantly lower the overall selectivity of the process. In addition, the process involves oxygen and hydrocarbons at a relatively high temperature and thus, the operating conditions need to be controlled in a strict way due to the explosive character of the mixture. H2O + O2 + O2 H H O OH
Figure 22. Schematic representation of the mechanistic model of the oxidative dehydrogenation of ethylbenzene to styrene, where A and B represent respectively acid and basic sites.
The numerous work realized by Figueiredo and co-workers,57,58,59have really helped
to establish that carbonyl/quinone functional groups present on the carbon surface are the active sites for performing ODH (Figure 22). A redox mechanism of Mars van Krevelen type involving quinone/hydroquinone groups on the surface of the activated carbon is suggested for the reaction, where the quinone surface groups are reduced to hydroquinone by the
